Get the image size in the web page by downloading only a small amount of data in the image header

Question

A web page pointed to by a url contains many images(gif,png,jpg...), I just want to find the one with the largest size(eg: choose 1024x600 instead of 800x300) by Swift code.

Currently, I have to download all images locally and then get their sizes

I know that the size data of the image is actually only stored in its header, does this mean that I only need to download the small header data of the image to get its size?

If possible, how can I parse to the size of the image through its header data?

preferably Swift or C code, thanks! ;)

Or is there a better way to get the size of the image from web without downloading it locally? Please enlighten me, thank you!

Update:

I find some ruby code to do this:

PNG:

IO.read('image.png')[0x10..0x18].unpack('NN')
=> [713, 54]

GIF：

IO.read('image.gif')[6..10].unpack('SS')
=> [130, 50]

BMP：

d = IO.read('image.bmp')[14..28]
d[0] == 40 ? d[4..-1].unpack('LL') : d[4..8].unpack('SS')

Do these codes work? Thanks.

Answer 1

You can just create an URLRequest and set the httpMethod to "HEAD". Then you can call an URLSession dataTask with your request and check the expectedContentLength property of the response:

import UIKit
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true

let link = "https://i.stack.imgur.com/varL9.jpg"
let url = URL(string: link)!
var request = URLRequest(url: url)
request.httpMethod = "HEAD"
URLSession.shared.dataTask(with: request) { data, response , _ in
    guard let response = response, (response as? HTTPURLResponse)?.statusCode == 200 else { return }
    DispatchQueue.main.async() {
        print("data", data)                                                                // "data 0 bytes\n"
        print("suggestedFilename:", response.suggestedFilename ?? "no suggestedFilename")  // "suggestedFilename: varL9.jpg\n"
        print("expectedContentLength:", response.expectedContentLength)                    // "expectedContentLength: 60807\n"
    }
}.resume()